Question

1. Explain how the amphipathic nature of membrane proteins helps form the three-dimensional structure of a single-pass protein like a receptor, versus a multipass protein like a channel.

2. Describe the different ways by which cells confine proteins or restrict protein movement to a specific region of the plasma membrane

Answer 1

1.

• The transmembrane proteins are amphipathic in nature. It means they have both hydrophobic and hydrophilic regions.
• Their hydrophobic region pass through the membrane, interact with the hydrophobic tail of the lipid molecule in the membrane. The hydrophilic regions of the protein are exposed on either side of the membrane.
• In single pass protein, the polypeptide passes the membrane only one time. In the multipass protein, the polypeptide passes through the membrane for multiple time.
• The peptide bond in the polypeptide chain that spans the membrane form hydrogen bond. This is because the peptide bonds are polar and there is no water molecule. This hydrogen bonding between the peptide bonds results in the formation of alpha-helical structure.
• This hydrogen bonding in multipass protein helps in forming beta sheets of polypeptide chains. These beta-barrels are rigid compared to an alpha helix. The beta-sheets structure forms the pore or channels through which various molecules can pass in and out of the lipid bilayer.
• In both types of protein topology, the hydrophobic peptide chain gives the stabilized protein structure.
• The arrangement of hydrophobic chains in the lipid bilayer provides van der Waals interaction. Thus the cylindrical shape of the proteins does not affect the lipid chain arrangement.
• The hydrophilic side chains of the protein get arranged in such a way that stabilizes the helical structure.

2.

• The lipid bilayer is a two-dimensional fluid. All transmembrane proteins freely float in the continuous lipid bilayer.
• Many cells have different ways of protein confinement to specific regions in the membrane.
• For example, The epithelial cells in the gut lining and kidney tubules, certain enzymes and transport proteins are restricted to apical membrane and other proteins are confined to lateral and basal membrane.
• The different ways of confining the lateral mobility of certain membrane proteins are:

1. Self-assembly of protein into aggregates. For example bacteriorhodopsin in the membrane of Halobacterium.
2. Membrane proteins can interact with the surface proteins of another cell.
3. Proteins can be tethered by interacting with macromolecular assemblies inside the cell or outside the cell.